A common principle for the measurement of fluid flow is the measurement of differential pressure over a restriction, like an orifice, a Venturi tube, or a so-called V-cone [1]. A differential pressure measurement can be a stand alone measurement for obtaining a measure of the flow, or it can be used in combination with other measurements, for example as in a WGM (Wet Gas Meter) described by this applicant in ref. [2], a reference which is hereby incorporated in full by way of reference, where differential pressure measurement is combined with a microwave resonator based measurement of the permittivity of the fluid, and where so-called PVT calculations are performed in order to arrive at a result indicating properties of the fluid and its composition.
Ref. [2] describes a number of embodiments of an integrated mechanical structure for creating a differential pressure (dP) in a pipeline. The differential pressure generating structure forms an integrated part of a microwave resonator.
Traditionally, a pair of pressure sensing taps is arranged at different locations in relation to a differential pressure generating structure in a conduit or pipeline for a fluid flow. The taps are normally connected to respective pipes which transfers the pressures at each respective pressure tap to respective inputs of a commercial differential pressure (dP) transmitter. In the dP-transmitter a sensor element, usually some form of membrane, is deformed by the differential pressure.
In one known differential pressure transmitter [3] the difference in pressure between two chambers of a differential pressure transmitter is detected by arranging a diaphragm between the two chambers and detecting the deflection of the diaphragm by one or more contact-free inductive pickups. Electrical signals from the pickups are used as indicators of pressure changes.
In other prior art solutions for pressure measurement a flexible membrane is arranged as part of a capacitor [4]. The capacitance thereby changes with the deformation of the membrane. An electronic capacitance measuring circuit may then be connected to the parallel-plate capacitor for converting the capacitance of the capacitor into an electric signal representing the value of the capacitance of the capacitor and of the associated parts.
Another means of measuring the deflection, and thus the differential pressure is to use strain gauges attached to the membrane. By measuring the change of resistance in the strain gauges, the differential pressure over the membrane can be found.
The prior art has some limitations. Measurements of typical inductance, capacitance and resistance values are prone to drift, e.g. with temperature of the electronics, unless properly compensated. Contributions to the measured capacitance from lengthy wires and other parts of the measurement circuitry may interfere with the desired measurement. In the design of a traditional differential pressure measurement system a design compromise typically therefore normally to be made between a low interference solution and a short distance between sensor transducers and electronics units.
In subsea applications prior art dP-measurement techniques have a couple of other additional limitations to overcome. First, prior art dP-units qualified for subsea use are relatively expensive and relatively bulky, and they add to the total amount of electronics in a flow metering installation. A reduced MTTF (Mean Time to Failure) typically results. The dP-units typically also require transmitter electronics for communication with other units receiving the measurement results, e.g. a computing unit in a Wet Gas Meter (WGM). In subsea applications, where the meter is difficult to access for service, it is desirable to have an MTTF-value that is as high as possible. To achieve a high MTTF-value it is generally desirable to reduce the number of electronic units.
Secondly, when used in the oil and gas industry for transporting fluid mixtures of liquid and gaseous hydrocarbons and possibly water, a dP-transmitter is normally connected to pressure taps via a pipe or duct section of some length. Such pipe or duct sections are prone to clogging, e.g. by hydrate formation due to a higher temperature inside the transporting conduit than in the pipes leading to the dP-transmitter. This is especially true in subsea applications where the ambient seawater cools the pressure tapping ducts to a lower temperature than the flow.
Hence, it is an objective of the present invention to provide a new apparatus for differential pressure measurement which is less affected by the above limitations, and whose electronic part is simplified as compared to the present solution.